Understanding Bicycle Gears: More Than Just Speed & Hills

Question
I know that bicycle in the front 3 gear & back 7 gear. What else I can use the gear beside cycling fast & going up slope? I am not use to the gear Pls help me.  

Answer
Hello,
You can also use it to workout (keep it in the small gear in the front) so you peddle alot more = burning calories and to stop and start biking again, almost like a transmition, you down and up shift (which also prevents the chain from stretching).

Hope this answers your question!

Thanks!
George